This invention relates generally to optical switches and more particularly, to an optical switch having a movable wedge or a plurality of movable wedges, which serve to steer a beam of light.
Optical switches of various kinds are well known for selectively switching light from a waveguide, such as optical fibre or light-conducting path, to another.
To fulfill this requirement, it has been well known to provide 2xc3x972 optical switches having two ports on each side, wherein the switch is configurable to make a connection between ports 1 and 2 and simultaneously to provide a connection between ports 3 and 4. Alternatively, such switches are configurable to provide simultaneous connections between ports 1 and 4, and ports 3 and 2. Hence these prior art switches have two states; a first state wherein two bar connections are formed and a second state wherein 2 cross connections are formed. Providing suitable coupling in both switching states, and providing a switch that is fast enough, and tolerant of physical disturbances is a daunting task most switch manufacturers face.
A well known optical switch made by JDS Fitel Inc. has been sold in the United States since Feb. 11, 1992 under the product number SR22xx-ONC. This optical switch includes a pair of GRaded INdex (GRIN) lenses having a reflector or mirror that can be selectively disposed therebetween. Each GRIN lens has two ports offset from the optical axis (OA) of the lens.
In a graded index medium that has a refractive index that varies with position, optical rays follow curved trajectories, instead of straight lines. By judicious selection of the refractive index, a GRIN rod can behave like a conventional optical element such as a prism or a lens. Lenses of this type are produced under the trade name xe2x80x9cSELFOCxe2x80x9d; the mark is registered in Japan and owned by the Nippon Sheet and Glass Co. Ltd. GRIN lenses are used extensively as a means of coupling optical signals from one waveguide such as an optical fiber, to another, for example, in optical switches. The use of GRIN lens provides a number of advantages over other conventional lenses. For example, GRIN lenses are relatively inexpensive, compact, and furthermore have parallel flat end faces. In particular, the flat end face of the GRIN lens allows a single lens to be used as a means of collimating or focusing light.
An optical arrangement is shown in FIG. 1, wherein two quarter pitch GRIN lenses 10aand 10b are disposed so that their collimating ends are adjacent one another in a back-to-back relationship. A very thin optical element in the form of a filter 12 is sandwiched therebetween. The filter 12 can be coated directly on one of the inwardly facing end faces of the lenses, or alternatively may be coated on a substrate that is anti-reflection coated and sandwiched between the two GRIN lenses 10a and 10b. It should be noted, that the optical axes of the input/output fibres 11a and 11b are parallel with the optical axes of the two GRIN lenses. Since the beam traversing the lenses 10a and 10b about the filter element 12 is at a location substantially coincident with the optical axes of the GRIN lenses, the light input orthogonal to the end face of the lens 10a at port P1, propagates through the filter 12 and through the second lens 10b and exits at port P2 as a focused beam that is parallel to the input beam and the optical axes of the lenses 10a and 10b. 
FIG. 2 illustrates an offset that occurs when a gap is present between a pair of coaxial GRIN lenses 12a and 12b. The beam exiting the lens 12a intersects the end face equidistant from the optical axis indicated by lines 20a and 20b, which define the outer most limits of the beam as it traverses the lens 12a end face. However, due to the gap between the lenses 12a and 12b, the beam traverses the inwardly facing end face of the lens 12b having its outermost limits defined by the locations 22a and 22b which are not equidistant from the optical axis OA of the second lens 12b. This beam shift downward results in the output beam being directed upward along the optical axis of the optical fibre 14b. Accordingly, substantial coupling losses may occur between an input port on a first GRIN lens and an output port on a second GRIN lens, when the input and output ports are disposed adjacent the optical axes of the two GRIN lenses, and wherein a gap separating the GRIN lenses causes a beam propagating from the input port through the first GRIN lens to be shifted as it traverses the element towards the output port and enters the second lens at an offset to the optical axis of the lens. To overcome this disadvantage and to provide a more efficient optical coupling, the fibre 14b is provided at an angle xcex8 greater than 0 degrees with respect to the optical axis of the lens.
It is also possible, as shown in FIG. 3, to launch the beam 30 at a judiciously selected angle xcex8S at the left input end face of the GRIN lens 16b in such a way that the beam is selectively directed towards a desired output port location at the right output end face of the GRIN lens 16b. Moreover, by ensuring that the beam has its centre substantially coincident with the optical axis OA of the lens, the beam thus propagates through the lens 16b and exits the output end of the lens parallel to the OA of the lens. From a manufacturing standpoint, when using GRIN lenses in switches or routers, it is preferable to use a transmissive switching optical element, in which zero or a number of internal reflections in each plane, and/or any number of refractions, are imposed on the incident light between the lenses rather than a reflective element imposing one reflection, to route, shift, or direct a beam from one port to an alternate port when the element is disposed between lenses. Thus, by providing a transmissive element such as a prism, the switch is much less sensitive to angular deviation and misalignment of the element than a switch using a reflective element such as a mirror. For example, in comparing angular sensitivity based on a 0.05 dB excess insertion loss criterion, an existing single mirrorbased switch has a typical angular tolerance of 0.007 degrees. An existing prism-based switch has an angular tolerance of 0.03 degrees, whereas the transmissive optical wedge-based switch described in accordance with this invention has an angular tolerance of 1.4 degrees.
It is an object of the instant invention to provide an improved optical switch having a transmissive wedge movable between two GRIN lenses for changing the angle of the collimated beam by a selected amount so that the output beam exits the output end face substantially parallel to the optical axis of the GRIN lenses, regardless of the connect state.
It is an object of this invention to provide a relatively inexpensive and easy to manufacture switch that will serve as an Nxc3x97M optical switch.
In accordance with this invention, there is provided an optical switch comprising:
at least one input port on one side for launching an optical signal along an input optical path;
at least two output ports on an opposite side for receiving the optical signal, a first of the at least two output ports optically coupled to the at least one input port; and
a light transmissive wedge having at least two non-parallel surfaces, the wedge movable into and out of the input optical path, the wedge movable at least between first, second, and third positions corresponding to first, second, and third connect states, respectively.
In accordance with this invention, there is provided a method for switching a beam of light from one of a plurality of output ports to another, comprising the step of:
receiving at an input port a beam of light parallel to the optical axis of a first GRIN lens, the first GRIN lens for collimating the beam of light;
transmitting the optical signal along an optical path to a second GRIN lens optically coupled to the plurality of output ports, the second GRIN lens for focussing the beam of light; and
inserting a wedge in the optical path for modifying the optical path so that the signal switches from one of the plurality of output ports and so that the beam of light exits the second GRIN lens at a predetermined output port of the plurality of output ports substantially parallel to the optical axis of the GRIN lens.